The present invention relates to a method of forming silicon nitride film, and particularly to a method of forming silicon nitride film by a low-pressure CVD method maintaining a high film-forming efficiency.
The silicon nitride film (Si.sub.3 N.sub.4) is used for semiconductor devices to form, for example, a mask for selectively oxidizing silicon, to form a surface protection film and to form a memory in formed MNOS (Metal Nitride Oxide Semiconductor) structure. The silicon nitride film is, usually, formed by the CVD (chemical vapor deposition) method using a silane-type gas and ammonia. Preferably, however, a so-called CVD method is employed in which the pressure during the reacting is maintained lower than the normal pressure (one atmosphere=760 Torr). Namely, mean free paths and diffusion constants of the reaction gas and carrier gas increase with the decrease in the pressure. Therefore, thickness of the silicon nitride film formed on the surface of the wafer and film characteristics such as reflection factor and the like are distributed more uniformly at reduced pressure than at normal pressure. Further, as the mean free path and the diffusion constant increase, films are uniformly formed on a number of wafers that are vertically arranged in the reaction furnace, minimizing variance in the quality of films among the wafers, thereby increasing processing capacity since a large number of wafers are processed simultaneously.
U.S. Pat. No. 4,279,947, the contents of which are incorporated herein, discloses a method of depositing a silicon nitride film on a wafer by the low-pressure CVD method, in which the temperature in the reaction furnace is maintained at about 650.degree. C. to about 1000.degree. C., and a silane gas reacts with ammonia in the reaction furnace under a pressure of about 0.3 to about 10 Torr to obtain good results.
The inventors of the present invention have conducted experiments extensively relying upon the above-mentioned data and have obtained the results as described below. In such experiments, the wafers had a uniform spacing between adjacent (opposed) surfaces of adjacent wafers throughout the total number of wafers, with such spacing being 5 mm. That is, 100 silicon wafers having a diameter of about 75 mm (about three inches) were simultaneously treated under the above-mentioned conditions (about 750.degree. C., 0.3 Torr) to obtain silicon nitride films that were nearly 100% satisfactory (that is, nearly 100% of the films did not vary in thickness by more than 5% over the entire wafer). However, when silicon wafers having larger diameters, i.e., having diameters of 100 mm, 125 mm, and 150 mm were treated, nonuniform portions developed in the films, and the number of wafers that were satisfactory (i.e., did not vary in thickness over the entire wafer by more than 5%) decreased to 95, 75, and 50. Namely, as the diameter of the silicon wafers increases, satisfactory results are not necessarily obtained under the above-mentioned conditions. The recent trend toward production of large wafers makes it necessary to modify the low-pressure CVD method.